To meet the requirements of multidimensional spectral detection and visible-light imaging of flight targets, a common-aperture optical system based on the Cassegrain telescope was designed. The front end of the system was composed of bi-reflective system. The primary mirror is a paraboloid and the secondary mirror is a hyperboloid. The light beam in the back end was split by tilted flat plates and received by the following subsystems. The problem of astigmatism caused by tilted flat plates was solved successfully by utilizing two cylindrical lenses in the visible-light imaging system. The deviation of the optical axis caused by the tilted flat plate was compensated by adding reversed tilted flat plates. Visible-light imaging and mutispectrum (200-400 nm, 400-760 nm and 760-2 500 nm) reception could be realized for flying targets with diameters less than 0.5 m within 0.5-1.5 km. The MTF values of each field of view in the imaging module were all greater than 0.5 at a Nyquist frequency of 35 lp/mm. This result is extremely close to the diffraction limit curve. All coupling modules meet the coupling requirements of optical fibers. Through athermal design, the common-aperture system can work normally in the temperature range from -20 ℃ to 50 ℃. The tolerance analysis results show that the system can meet the requirements of manufacture, installation and adjustment.